Bipolar transistors are important components in, for example, logic circuits, communications systems, and microwave devices. A bipolar transistor is essentially a three terminal device having three regions of alternating conductivity type which are commonly referred to as the emitter, base, and collector regions. Since their invention in 1947 at Bell Telephone Laboratories, Incorporated, many proposals have been made in attempts to improve the performance of such transistors with respect to their, for example, speed, power, and frequency characteristics. In particular, many proposals have been made which would improve the performance of such devices so that they would be better suited for high speed applications such as microwave and logic devices.
One such proposal is a transistor with a wide bandgap emitter which is discussed in, for example, RCA Review, 18, pp. 332-342, Sept., 1957. A transistor with a wide bandgap emitter has an emitter region in which the bandgap is greater than the bandgap in the base region. An essential feature of the wide bandgap emitter transistor is that the bandgap difference between the emitter region and the base region suppresses hole injection. The result of the suppression is that the wide bandgap emitter transistor has a higher injection efficiency than does a transistor having a constant bandgap in the emitter and base regions. Consequently, the base region may be more heavily doped than the emitter region. As a result, the device has both a lower base resistance and emitter-base capacitance than does a constant bandgap transistor. Both of these characteristics are desirable for high frequency operation but neither one adversely effects the high emitter injection efficiency.
Transistors having wide bandgap emitters are commonly referred to as heterojunction bipolar transistors and have several advantages over, for example, metal semiconductor field effect transistors. The advantages include a larger current-drive capacity per unit area, a smaller sensitivity of switching speed to output loading, and a nearly fixed turn-on voltage.
The above-mentioned paper by Kroemer in the RCA Review also mentioned the idea of introducing a quasielectric field in the base region of the transistor. A quasi-electric field is one which produces forces on the electrons and holes that are not equal. The quasielectric field results from a crystalline nonuniformity and is conveniently introduced by having a graded bandgap base region. The quasi-electric field reduces the transit time of minority carriers in the base region and leads to an increase in device operating speed.
Other structures for increasing carrier velocity and reducing carrier transit time are known. For example, IEEE Electron Device Letters, EDL-3, pp. 407-408, December 1982, teaches repeated velocity overshoot structures. The structures use, for example, graded bandgap regions of thin acceptor and donor regions to achieve velocity overshoot.
The effect of the reduced carrier transit time in the base region was first demonstrated by F. Capasso et al and was disclosed in Applied Physics Letters, 42, pp. 93-95, 1983. The device disclosed by Capasso et al was a phototransistor having a wide bandgap emitter and a graded bandgap base region. The device had a response time less than approximately 20 psec. Later studies showed that the electron velocity was as high as 1.8.times.10.sup.7 cm/sec in a graded bandgap p.sup.+ AlGaAs layer. The layer was 0.4 .mu.m thick and the quasi-electric field was 8.8.times.10.sup.3 V/cm.
Other heterojunction bipolar transistors have been disclosed in literature. For example, Applied Physics Letters, 40, pp. 816-818, May 1, 1982, discloses such a device which had a constant bandgap base region and was fabricated from the GaAlAs-GaAs materials system using liquid phase epitaxy. High current gains were reported, .beta. greater than 2000, with devices having relatively low emitter doping, less than approximately 10.sup.16 /cm.sup.3, and a highly doped base, approximately 10.sup.18 /cm.sup.3. The device also had a relatively high transition frequency, 1.3 GHz, which was obtained with a collector current of 1 mA.
Another heterojunction bipolar transistor was described in Electronics Letters, 18, pp. 25-26, Jan. 7, 1982. This device was a double heterojunction transistor which had a wide bandgap collector region as well as a wide bandgap emitter region. The double heterojunction device disclosed eliminated the turn-on voltage of prior devices which was caused by the difference between the built-in voltages of the emitter base heterojunction and the base-collector homojunction.